A variety of aqueous electroplating bath compositions and processes for electrodepositing a nickel-iron alloy on electrically conductive substrates are known in the art and are in widespread commercial use. Nickel-iron electrodeposits because of their excellent corrosion resistance are particularly useful for providing decorative finishes on corrosion susceptible substrates over which a subsequent electrodeposition of chromium is applied. In order to achieve satisfactory nickel-iron deposits for decorative purposes it is extremely important that such electrodeposits are characterized by their high-leveling properties, brightness and good ductility and that uniformity in these beneficial characteristics are achieved over the entire electrodeposit.
Typical of nickel-iron electroplating bath compositions and processes are those described in prior U.S. Pat. Nos. 3,354,059; 3,795,591; 3,806,429; 3,812,566; 3,878,067; 3,974,044; 3,994,694; 4,002,543; and 4,089,754. The majority of the aforementioned U.S. patents are directed to nickel-iron electroplating compositions and processes for electrodepositing decorative nickel-iron deposits on conductive substrates and incorporate various additive agents and combination of additives for increasing the leveling of the deposit and to increase brightness. While certain of the nickel-iron plating bath compositions have provided for satisfactory electrodeposits for use in decorative applications, there has been a continuing need for improved bath compositions which provide for still further improvements in the leveling characteristics and brightness of the deposit formed. The use of selected primary and secondary brighteners and combination of brighteners of the types heretofore known in such electroplating baths have enhanced the brightness of the deposit obtained but their effectiveness tends to peak out before an electrodeposit of super brightness and leveling can be obtained.
The choice of complexing agents used to stabilize the iron has also been recognized as an important factor on the brightness and leveling of the electrodeposits obtained. For example, a citrate complexing agent not only complexes the iron ions present but also the nickel ions in the bath. Because of the presence of the nickel citrate complex, the resultant brightness and leveling of the electrodeposits is at best average. The use of gluconates as a complexing agent provides the advantage that it does not complex nickel and therefore somewhat better leveling is obtained. However, the iron gluconate complex possesses characteristics which somewhat restrict the leveling of the electrodeposit obtained.
Attempts to increase the operating pH of the bath has resulted in some improvement in the leveling and brightness of the electrodeposit obtained. However, at such higher pH levels, an increase in the ferric ion concentration occurs rendering the operating bath very sensitive to high iron concentrations and to the organic addition agents present detracting from efficient use and simple control of the operating bath.
The use of reducing saccharides in combination with selected complexing agents such as those disclosed in U.S. Pat. No. 3,974,044 has been found to result in good leveling and brightness of the nickel-iron alloy deposit with low sensitivity to high iron concentrations and the presence of organic additives, such as secondary brightener additives. The use of tartrates instead of the complexing agents disclosed in U.S. Pat. No. 3,974,044, results in significantly improved leveling and brightness of the nickel-iron alloy deposit but there is a marked increase in the sensitivity of the bath to iron and organic additives. A further problem is evidenced by an apparent interference of such compositions with the buffering additives employed causing the pH of the bath to rise very rapidly during use requiring constant pH adjustment and the associated expense and difficulty of such control to maintain the bath within satisfactory operating parameters.
The present invention provides for a further improvement in nickel-iron electroplating bath compositions and processes by overcoming many of the problems and disadvantages associated with prior art compositions and techniques while at the same time attaining nickel-iron electrodeposits which are characterized by their extremely high-leveling and brightness characteristics. The bath composition and process of the present invention is further characterized by its ability to achieve extraordinary brightness and leveling over a broad pH range even when depositing nickel-iron alloys containing 35% iron and higher while simultaneously providing a bath of reduced sensitivity to iron concentration and to the presence of high concentrations of secondary organic addition agents.